AoE2 WITH the expansion pack = AOC (not AOK).
I love AOC best of all of the AOE series. I don't care for the campaigns of AOE1/AOK/AOC/AOE2 at all, but I've tied a couple of each. So I don't buy these games for the campaign mode and I won't comment on those differences, if any.
I play strictly for multiplayer. I loved the MP of AOC so long as it was using direct IP, because I felt that MSN game zone pretty much stunk fully. I wish AOE3 had the ease of play using TCP/IP for direct play that AOC had, but I haven't gotten it to work once yet, which also stinks completely. Play on ESO is only somewhat better than it was on MSN game zone, because it does work intermittently.
As for game play, these are the things I don't like about AOE3: I can't garrison troops in Forts, Barracks, Stables & Towers. I don't like build limits on ships & troops. I don't care much for earning XP, Home Cities, Explorers, digging for treasures, fighting guardians, & having only ONE login account per CD-key (I have teenagers). I also liked the full randomness of AOC maps. AOE3 maps get boring FAST because they are always the same-same-same (yawn).
I do like the trade posts, Indian alliances, & high graphics quality. I like that there are ships you can build troops with (galleons). I like some of the features that you get from cards, such as the hot air balloon.
All in all, AoE3 would be a much better game if it were pretty much exactly like AOC except with the cranked up graphics, the new civs, new units, new ships, new things to research, etc.
I hope AOE4 is more like AOC than AOE3. It would be nice if it was concerning the US war of Independence from England, Confederate Independence from the Yankees, WW1, WW2, etc. I'd love to replay the "civil war" with this engine and make the good guys win this time. JMHO.